5 ms·

Anyone know why the TCAS only instructs the two approaching aircraft to change altitude only, but not heading?

by laydn 3y ago

ceejayoz 3y ago
https://aviation.stackexchange.com/questions/36451/why-does-tcas-not-issue-turning-commands-as-resolution-advisory https://aviation.stackexchange.com/questions/36451/why-does-...
MerelyMortal 3y ago
I met someone who says he helped develop TCAS, and that it decides which one to climb or descend based on the registration number. I'm guessing that it does not issue turn instructions because it only gives a Resolution Advisory when a collision is imminent (otherwise they would be giving RAs all the time). At altitude, where the air is thinner, it is much easier for a plane to immediately climb or descend, planes turn very slowly when the air is thin.
- ceejayoz 3y ago
  That's only partially true, in the case of a tie. https://aviation.stackexchange.com/questions/92495/how-does-the-tcas-decide-which-aircraft-to-climb-and-which-to-descend https://aviation.stackexchange.com/questions/92495/how-does-... > But what if the other aircraft is "thinking" the same thing? That's the heart of this question. The answer is, whichever TCAS makes a decision first, and then in case of a tie, the lower Mode S address "wins":
  - jll29 3y ago
    Thanks for answering my question, namely how does the collision resolution in TCAS work? (Different from, say, TCP, which uses randomness) EDIT: FAA PDF on TCAS 7.1 - https://www.faa.gov/documentlibrary/media/advisory_circular/tcas%20ii%20v7.1%20intro%20booklet.pdf https://www.faa.gov/documentlibrary/media/advisory_circular/... This story is a moving example of the self-improvement going on in global air traffic industry; perhaps sometimes slowly, but definitely self-improving. The following book compares the transparency of the air traffic industry with the lack of transparency that currently exists in the medical sector, where errors are denied: https://www.amazon.com/Black-Box-Thinking-Psychology-Paperback/dp/0111254744 https://www.amazon.com/Black-Box-Thinking-Psychology-Paperba...
lisper 3y ago
Good answers in sibling comments, but for those who don't want to grovel through a lot of stackexchange answers here is the TLDR: There are two reasons: 1. When TCAS was originally developed, the direction information to the other aircraft was not very accurate, being obtained only via the TCAS antenna. Nowadays GPS information is transmitted via ADS-B and so it's much more accurate, but taking advantage of this would require a major redesign. It may happen eventually, but... 2. Aircraft can change altitude faster than they can change heading. Also, most aircraft can change pitch faster than they can roll, and they are longer and wider than they are high. So effecting enough altitude change to avoid a collision can be done faster than effecting enough heading change to do so.
- sokoloff 3y ago
  ADS-B is not guaranteed to be available from all aircraft in most of the airspace below 10K feet MSL. (Basically, if a transponder used to be optional before, ADS-B out is also optional.) Mode-C (altitude encoded transponder replies) are also not required, but the equipage rate is very high at this point, which is less the case for ADS-B among GA aircraft. https://www.aopa.org/go-fly/aircraft-and-ownership/ads-b/where-is-ads-b-out-required https://www.aopa.org/go-fly/aircraft-and-ownership/ads-b/whe...
  - lisper 3y ago
    > ADS-B is not guaranteed to be available from all aircraft in most of the airspace below 10K feet MSL. Sure, but so what? The lack of universality is not an impediment to implementing a new TCAS system that uses ADS-B data when it's available. (And BTW, the places where ADS-B is not required are generally places where traffic is sparse and so the risk of a collision is pretty low to begin with.)
    - sokoloff 3y ago
      If the expense and lead time for the redesign now that more reliable and precise position (and first derivative thereof) are available from GPS via ADS-B are to be considered, it's fair to ask "well, what are the downsides as compared to using altitude for separation as today?" and one of them is "a higher percentage of aircraft are transmitting mode-C than will be transmitting ADS-B out". Given that the clear priority is now to follow a TCAS RA (resolution advisory) over an ATC instruction, I think that the current TCAS approach is good enough to cause aircraft to miss each other and that a GPS-based redesign is unlikely.
sgustard 3y ago
You could imagine if, in addition to obeying the altitude change, every plane began to turn rapidly to the right, it could only help the situation? As long as we all agree to turn right in case of a collision warning?

Anyone know why the TCAS only instructs the two approaching aircraft to change altitude only, but not heading?

by laydn 3y ago

Anyone know why the TCAS only instructs the two approaching aircraft to change altitude only, but not heading?

https://aviation.stackexchange.com/questions/36451/why-does-tcas-not-issue-turning-commands-as-resolution-advisory https://aviation.stackexchange.com/questions/36451/why-does-...

MerelyMortal 3y ago

I met someone who says he helped develop TCAS, and that it decides which one to climb or descend based on the registration number. I'm guessing that it does not issue turn instructions because it only gives a Resolution Advisory when a collision is imminent (otherwise they would be giving RAs all the time). At altitude, where the air is thinner, it is much easier for a plane to immediately climb or descend, planes turn very slowly when the air is thin.

ceejayoz 3y ago
That's only partially true, in the case of a tie. https://aviation.stackexchange.com/questions/92495/how-does-the-tcas-decide-which-aircraft-to-climb-and-which-to-descend https://aviation.stackexchange.com/questions/92495/how-does-... > But what if the other aircraft is "thinking" the same thing? That's the heart of this question. The answer is, whichever TCAS makes a decision first, and then in case of a tie, the lower Mode S address "wins":
- jll29 3y ago
  Thanks for answering my question, namely how does the collision resolution in TCAS work? (Different from, say, TCP, which uses randomness) EDIT: FAA PDF on TCAS 7.1 - https://www.faa.gov/documentlibrary/media/advisory_circular/tcas%20ii%20v7.1%20intro%20booklet.pdf https://www.faa.gov/documentlibrary/media/advisory_circular/... This story is a moving example of the self-improvement going on in global air traffic industry; perhaps sometimes slowly, but definitely self-improving. The following book compares the transparency of the air traffic industry with the lack of transparency that currently exists in the medical sector, where errors are denied: https://www.amazon.com/Black-Box-Thinking-Psychology-Paperback/dp/0111254744 https://www.amazon.com/Black-Box-Thinking-Psychology-Paperba...

lisper 3y ago

Good answers in sibling comments, but for those who don't want to grovel through a lot of stackexchange answers here is the TLDR: There are two reasons: 1. When TCAS was originally developed, the direction information to the other aircraft was not very accurate, being obtained only via the TCAS antenna. Nowadays GPS information is transmitted via ADS-B and so it's much more accurate, but taking advantage of this would require a major redesign. It may happen eventually, but... 2. Aircraft can change altitude faster than they can change heading. Also, most aircraft can change pitch faster than they can roll, and they are longer and wider than they are high. So effecting enough altitude change to avoid a collision can be done faster than effecting enough heading change to do so.

sokoloff 3y ago
ADS-B is not guaranteed to be available from all aircraft in most of the airspace below 10K feet MSL. (Basically, if a transponder used to be optional before, ADS-B out is also optional.) Mode-C (altitude encoded transponder replies) are also not required, but the equipage rate is very high at this point, which is less the case for ADS-B among GA aircraft. https://www.aopa.org/go-fly/aircraft-and-ownership/ads-b/where-is-ads-b-out-required https://www.aopa.org/go-fly/aircraft-and-ownership/ads-b/whe...
- lisper 3y ago
  > ADS-B is not guaranteed to be available from all aircraft in most of the airspace below 10K feet MSL. Sure, but so what? The lack of universality is not an impediment to implementing a new TCAS system that uses ADS-B data when it's available. (And BTW, the places where ADS-B is not required are generally places where traffic is sparse and so the risk of a collision is pretty low to begin with.)
  - sokoloff 3y ago
    If the expense and lead time for the redesign now that more reliable and precise position (and first derivative thereof) are available from GPS via ADS-B are to be considered, it's fair to ask "well, what are the downsides as compared to using altitude for separation as today?" and one of them is "a higher percentage of aircraft are transmitting mode-C than will be transmitting ADS-B out". Given that the clear priority is now to follow a TCAS RA (resolution advisory) over an ATC instruction, I think that the current TCAS approach is good enough to cause aircraft to miss each other and that a GPS-based redesign is unlikely.

sgustard 3y ago

You could imagine if, in addition to obeying the altitude change, every plane began to turn rapidly to the right, it could only help the situation? As long as we all agree to turn right in case of a collision warning?